1. Field of the Invention:
The present invention relates to a sum or differential frequency signal generating circuit for forming an output signal of the frequency of the sum or difference of the frequencies of two input signals.
2. Description of the Prior Art:
The present invention is used in a SSB (Single Side Band) modulator. FIG. 1 shows a constitution of a conventional SSB modulator, in which a reference numeral 1 denotes an input signal source of a sine wave signal of the frequency f.sub.1 and 2 indicates an input signal source of a sine wave signal of the frequency f.sub.2. One of these input signal sources 1 and 2 is used as a modulation signal of a predetermined band, while the other is used as a carrier signal source. Numerals 3 and 4 represent phase shifters for performing the phase shift of 90.degree.; 5 and 6 indicate multipliers; 7 an adder; and 8 an output terminal.
A first input signal and a second input signal obtained through the phase shifter 4 are supplied to the multiplier 5, while the second input signal and the first input signal obtained through the phase shifter 3 are supplied to the multiplier 6, and the outputs of the multipliers 5 and 6 are supplied to the adder 7. A cosine wave signal appears at the outputs of the phase shifters 3 and 4, while a sine wave output signal having a frequency of the sum of the frequencies of the two individual sine waves, (f.sub.1 +f.sub.2), appears from the adder 7 as shown in FIG. 2A. FIGS. 2A and 2B show the case where the first and second input signals each have a single frequency for the purpose of simplicity.
In addition, by inverting the phase of the output of the multiplier 5 and supplying it to the adder 7, a output sine wave signal having the frequency (f.sub.2 -f.sub.1), or the difference between the frequencies of the two individual sine waves appears at the output terminal 8 as shown in FIG. 2B. As obvious from FIGS. 2A and 2B, only the signal to be modulated (single side band) is obtained in the SSB modulator. For example, as shown in FIGS. 2A and 2B, assuming that the carrier frequency is f.sub.2 and the output to be modulated has a frequency near the carrier frequency f.sub.2, it is difficult to remove the carrier component by a filter; therefore, it is advantageous to use the SSB modulating method. More practically, in the case where a chrominance signal is low-frequency converted and a luminance signal is FM modulated and where FM modulation audio signals having different carrier frequencies on each of the adjacent tracks are inserted between those signals, the SSB modulator is used to convert the carrier frequencies of the FM modulation audio signals into the desired values. In this case, frequencies such as f.sub.1 =150 kHz and f.sub.2 =1.3 MHz are used.
As shown in FIG. 1, when the phase shifters 3 and 4, and the multipliers 5 and 6 are constituted by separate circuit blocks, the circuit arrangement and also the adjusting means for each circuit block and the amplitude adjusting means for the output of the multipliers 5 and 6 become complicated i.e., a total of four adjusting means were needed.
Firstly, there is a problem of the carrier leak such that the signal component of f.sub.1 or f.sub.2 appears at the outputs of the multipliers 5 and 6 in addition to the normal signal component of (f.sub.1 +f.sub.2) or (f.sub.2 -f.sub.1). In particular, it is difficult to remove the signal component (i.e., component of the frequency f.sub.2) close to the frequency (f.sub.1 +f.sub.2 or f.sub.2 -f.sub.1 in FIG. 2) of the output to be modulated among this carrier leak by a filter. To prevent this carrier leak, it was necessary to adjust the carrier balance of the multipliers 5 and 6.
Secondly, unnecessary components appear in the output signal even when phase shift amounts .phi..sub.1 and .phi..sub.2 of the phase shifters 3 and 4 have errors. The output signal is the signal obtained by adding the outputs of the multipliers 5 and 6 and is represented by the following expression:
K.sub.1 .multidot.sin.omega..sub.1 t.multidot.sin(.chi..sub.2 t+.phi..sub.2)+K.sub.2 sin.omega..sub.2 t.multidot.sin(.omega..sub.1 t+.phi..sub.1 =K.sub.3 .multidot. sin (.omega..sub.1 +.omega..sub.2)t. Each of the phase shift amounts .phi..sub.1 and .phi..sub.2 has to be 90.degree. in order to obtain the result K.sub.3 .multidot.sin(.omega..sub.1 +.omega..sub.2)t in the above expression. When there is an error, the unnecessary component of (.omega..sub.2 -.omega..sub.1) appears. It is difficult to remove this unnecessary component by a filter in the case where the frequency of the unnecessary component is close to the frequency of the output to be modulated.
Thirdly, when the levels of the output signals of the two multipliers 5 and 6 are not equal, the unnecessary component of (.omega..sub.2 -.omega..sub.1) is also produced. Coefficients K.sub.1 and K.sub.2 in the above expression denote the gains of the output signals of the multipliers 5 and 6. When (K.sub.1 .noteq.K.sub.2), the unnecessary signal component is generated. In a conventional circuit arrangement, the outputs of the multipliers 5 and 6 were balanced in the adder 7.